1. Field of the Invention
This invention relates to the preparation of microparticles. More particularly, the present invention relates to a method of encapsulating active agents to form controlled-release microparticles through the use of static mixers. The present invention also relates to a solvent system useful in a method of encapsulating active agents to form controlled-release microparticles. By "microparticles" or "microspheres" is meant solid particles that contain an active agent dispersed or dissolved within a biodegradable polymer that serves as the matrix of the particle.
2. Description of the Related Art
A variety of methods is known by which compounds can be encapsulated in the form of microparticles. It is particularly advantageous to encapsulate a biologically active or pharmaceutically active agent within a biocompatible, biodegradable, wall forming material (e.g., a polymer) to provide sustained or delayed release of drugs or other active agents. In these methods, the material to be encapsulated (drugs or other active agents) is generally dissolved, dispersed, or emulsified, using stirrers, agitators, or other dynamic mixing techniques, in a solvent containing the wall forming material. Solvent is then removed from the microparticles and thereafter the microparticle product is obtained.
An example of a conventional microencapsulation process is disclosed in U.S. Pat. No. 3,737,337 wherein a solution of a wall or shell forming polymeric material in a solvent is prepared. The solvent is only partially miscible in water. A solid or core material is dissolved or dispersed in the polymer-containing solution and, thereafter, the core-material-containing solution is dispersed in an aqueous liquid that is immiscible in the organic solvent in order to remove solvent from the microparticles. The substances to be encapsulated or embedded are dissolved or dispersed in the organic solution of the polymer (phase A), using conventional mixers including (in the preparation of a dispersion) vibrators, and high speed stirrers, etc. The dispersion of phase (A), containing the core material in solution or in suspension, is carried out in the aqueous phase (B) again using conventional mixers, such as high-speed mixers, vibration mixers or even spray nozzles, in which case the particle size of the microgranulates will be determined not only by the concentration of phase (A) but also by the particle sizes obtained.
Another example of a process in which solvent is removed from microparticles containing a substance is disclosed in U.S. Pat. No. 3,523,906. In this process, a material to be encapsulated is emulsified in a solution of a polymeric material in a solvent that is immiscible in water and then the emulsion is emulsified in an aqueous solution containing a hydrophilic colloid. Solvent removal from the microparticles is then accomplished by evaporation and the product is obtained.
In still another process, as disclosed in U.S. Pat. No. 3,691,090, organic solvent is evaporated from a dispersion of microparticles in an aqueous medium, preferably under reduced pressure.
Similarly, U.S. Pat. No. 3,891,570 discloses a method in which microparticles are prepared by dissolving or dispersing a core material in a solution of a wall material dissolved in a solvent having a dielectric constant of 10 or less and poor miscibility with a polyhydric alcohol, then emulsifying in fine droplets through dispersion or solution into the polyhydric alcohol and finally evaporating the solvent by the application of heat or by subjecting the microparticles to reduced pressure.
Another example of a process in which an active agent may be encapsulated is disclosed in U.S. Pat. No. 3,960,757. Encapsulated medicaments are prepared by dissolving a wall material for capsules in at least one organic solvent, poorly miscible with water, that has a boiling point of less than 100.degree. C., a vapor pressure higher than that of water, and a dielectric constant of less than about 10; dissolving or dispersing a medicament that is insoluble or slightly soluble in water in the resulting solution; dispersing the resulting solution or dispersion to the form of fine drops in a liquid vehicle comprising an aqueous solution of a hydrophilic colloid or a surface active agent, and then removing the organic solvent by evaporation. The size of the fine drops is determined according to the stirring speed, the viscosity of the organic solvent solution containing the medicament and the wall material, and the viscosity and surface tension of the vehicle.
Tice et al. in U.S. Pat. No. 4,389,330 describe the preparation of microparticles containing an active agent by using a two-step solvent removal process. This two-step solvent removal process is advantageous because it results in microparticles having higher active agent loading and a higher quality than techniques in which solvent is removed in a single step. In the Tice et al. process, the active agent and the polymer are dissolved in a solvent. The mixture of ingredients in the solvent is then emulsified in a continuous-phase processing medium that is immiscible with the solvent. A dispersion of microparticles containing the indicated ingredients is formed in the continuous-phase medium by mechanical agitation of the mixed materials. From this dispersion, the organic solvent can be partially removed in the first step of the solvent removal process. After the first stage, the dispersed microparticles are isolated from the continuous-phase processing medium by any convenient means of separation. Following the isolation, the remainder of the solvent in the microparticles is removed by extraction. After the remainder of the solvent has been removed from the microparticles, they are dried by exposure to air or by other conventional drying techniques.
Tice et al., in U.S. Pat. No. 4,530,840, describe the preparation of microparticles containing an anti-inflammatory active agent by a method comprising: (a) dissolving or dispersing an anti-inflammatory agent in a solvent and dissolving a biocompatible and biodegradable wall forming material in that solvent; (b) dispersing the solvent containing the anti-inflammatory agent and wall forming material in a continuous-phase processing medium; (c) evaporating a portion of the solvent from the dispersion of step (b), thereby forming microparticles containing the anti-inflammatory agent in the suspension; and (d) extracting the remainder of the solvent from the microparticles.
WO 90/13361 discloses a method of microencapsulating an agent to form a microencapsulated product, having the steps of dispersing an effective amount of the agent in a solvent containing a dissolved wall forming material to form a dispersion; combining the dispersion with an effective amount of a continuous process medium to form an emulsion that contains the process medium and microdroplets having the agent, the solvent, and the wall forming material; and adding the emulsion rapidly to an effective amount of an extraction medium to extract the solvent from the microdroplets to form the microencapsulated product.
Bodmeier, R. et al., International Journal of Pharmaceutics 43:179-186 (1988), disclose the preparation of microparticles containing quinidine or quinidine sulfate as the active agent and poly(D,L-lactide) as the binder using a variety of solvents including methylene chloride, chloroform, and benzene as well as mixtures of methylene chloride and a water miscible liquid, such as acetone, ethyl acetate, methanol, dimethylsulfoxide, chloroform, or benzene to enhance drug content.
Beck, L. R. et al., Biology of Reproduction 28:186-195 (1983), disclose a process for encapsulating norethisterone in a copolymer of D,L-lactide and glycolide by dissolving both the copolymer and the norethisterone in a mixture of chloroform and acetone that is added to a stirred cold aqueous solution of polyvinyl alcohol to form an emulsion and the volatile solvents removed under reduced pressure to yield microcapsules.
Phase separation or non-solvent induced coacervation is a method which has also been employed to prepare microparticles comprised of a biodegradable polymeric matrix and a biologically active agent. Many of the published procedures for microencapsulation with lactide/glycolide copolymers employ solvent evaporation/extraction techniques, but these techniques are mostly suitable for water insoluble drugs because water soluble drugs may partially partition into the aqueous phase during the preparation process. The phase separation method, utilizes non-solvents for the polymer and in which hydrophilic active agents also are not soluble, is an efficient method of encapsulation for these active agents.
In a conventional phase separation method, a known amount of polymer, such as poly(lactide-co-glycolide), PLGA, with a monomeric ratio of lactide to glycolide ranging from 100:0 to 50:50, is dissolved in an appropriate organic solvent. The solid drug, preferably lyophilized and micronized, may be dispersed in the polymer solution, where it is insoluble or slightly soluble in the organic solvent. Alternaltively, the active agent may be dissolved in water, or in water which contains some additives, and emulsified in the polymer solution, preferably mainly by sonication, forming a water-in-oil emulsion. The resultant suspension or emulsion is then added to a reactor and addition of a first non-solvent is initiated at a predetermined rate. A turbine mixer installed in the reactor is used to provide moderate mixing. At the completion of the phase separation process, the mixture is transferred into a quench tank containing a second non-solvent to solidify the semi-solid microspheres. The hardened microspheres are collected by sieving and are washed and stored in a vacuum oven for further drying.
Very often the solvents used in the known microencapsulation processes are halogenated hydrocarbons, particularly chloroform or methylene chloride, which act as solvents for both the active agent and the encapsulating polymer. The presence of small, but detectable, halogenated hydrocarbon residuals in the final product, however, is undesirable, because of their general toxicity and possible carcinogenic activity. Thus, a need exists to revise the known microencapsulation processes using less toxic and acceptable alternative solvents.
With conventional techniques for the microencapsulation of biological or pharmaceutical active agents, such as those described above, the microparticles form when the solvent containing an active agent and a polymer is emulsified or dispersed in an immiscible solution by stirring, agitating, vibrating, or some other dynamic mixing technique, often for a relatively long period of time. Such dynamic mixing techniques have several drawbacks. For example, it is difficult to control the size of the resulting microparticles, or the distribution of sizes obtained. As a consequence, use of dynamic mixing also presents problems when preparing microparticles containing biological or pharmaceutical agents on a production or commercial scale. Particularly, production equipment includes a costly emulsion tank, including equipment to stir or agitate the fluids. One of the controlling factors for overall process time is the time required to form a homogeneous (uniform) emulsion. Increased batch sizes in larger tanks require a longer time to form the emulsion, resulting in a longer overall production process time. Longer exposure times of the active agent to process solvents and to polymer solutions can lead to degradation or deactivation of the active agent. Scale-up to a production process from a laboratory emulsion process is particularly difficult for microencapsulation of biological or pharmaceutical agents since, as the batch and tank size are increased, stir speeds and viscosities within the larger tank have to be empirically optimized by trial and error at each stage of the scale-up. Likewise, the phase separation technique is not easily converted into a process for producing commercial scale quantities of microparticles because processing parameters, i.e., rate of non-solvent addition, agitation conditions, and the viscosity of both the active agent/polymer solution and the non-solvent must be empirically optimized by trial and error at each stage of scale-up. Thus, scale-up of conventional microencapsulation techniques is not only time consuming, but imprecise.
Tests were conducted in an attempt to scale-up a laboratory emulsion formation process from small stirred glass reactors to production equipment for microparticles containing estradiol benzoate. The shear created by the mixer blades determined the particle size of the emulsion; the higher the shear, the smaller the particles. Due to the low viscosity of the oil (organic) phase in the estradiol benzoate process, low shear is required to produce the large emulsion particles which were desired. In large reactors it is difficult to maintain low shear and still provide uniform mixing. The speed at which the agitator must turn to provide a uniform tank composition produces a small particle size with a broad distribution of sizes. Larger mixing blade diameters and multiple mixing blades, along the shaft helped to provide better mixing at low shear but still produced a very broad distribution of sizes. Particle size control became less reliable as batch size was increased.
Accordingly, one advantage of the method of preparing microparticles of the present invention is that accurate and reliable scaling from laboratory to commercial batch sizes can be done, while achieving a narrow and well defined size distribution of microparticles containing biologically or pharmaceutically active agents. This can be achieved for any suitable encapsulation technique including, but not limited to, solvent extraction and phase separation. A further advantage of the method of the present invention is that the same equipment can be used to form microparticles containing active agents of a well defined size distribution for varying batch sizes. Yet another advantage of the method of the present invention is that high quality microparticles having a high concentration of active agent can be obtained using a single step to remove solvent, or through a phase separation technique.